The overall objective of this project is to study the extent and nature of phosphorylation of normal and tumor (Na+ + K+)-ATPases. It has been noted that the specific activity of the (Na+ + K+)-ATPase of human adenocarcinoma is less than that of the normal kidney enzyme and is sensitive to inhibition by the antitumor proteins, macromomycin and largomycin. Phosphorylation of the normal kidney (Na+ + K+)-ATPase with muscle protein kinase both reduced the specific activity and increased the sensitivity to macromomycin such that it behaved similar to the tumor enzyme. These effects will be studied further with other (Na+ + K+)-ATPases from normaland tumor cells.